The invention relates to an interlocking nail, particularly for fractures of the proximal femur.
For a repair of trochanteric fractures and fractures of the femoral neck or femoral head, it is known to provide an interlocking nail which is driven into the femur from the proximal end and, in a an oblique through bore, guides a femoral neck pin which is introduced into the femoral head via the femoral neck of the femur. It is known to provide the femoral neck pin with a thread to allow it to be screwed into the femoral head (femoral neck screw); however, it is also known to design the femoral neck pin as a blade. In the proximal part of the interlocking nail, it is further known to provide a bore which interacts with the femoral neck pin in such a way that it can axially move in the oblique through bore, but cannot rotate. Such a nail is shown in U.S. Pat. No. 5,176,681.
The weight force of a patient who is provided with such an implant is essentially directed into the interlocking nail from the femoral neck pin. The nail cross-section, when under a load, is subjected to a combined stress which is composed of bending and tensile stresses. In case of an overload, a crack or fissure may develop, namely at the site at which the highest tensile stress occurs. The highest concentration of stress results on the sharp lateral edges on opposed sides of the femoral neck pin when it is in the oblique through bore. Thus, the so-called fatigue strength of the implant depends on the critical area having edges with a sharp-edged geometry. The highest tensile stresses occur at the screw inlet end of the bore.